The aging of the US population puts an increased proportion of the population at the ages of highest risk of the onset of musculoskeletal conditions and, among those with these conditions, at the ages of highest severity levels. The problem of aging is made more severe by the fact that many major chronic diseases are more prevalent in late middle age and among the elderly. In fact, most of the latter group has two or more chronic diseases. The impact of comorbidity is reflected in the cost data presented in this volume. Not only are the incremental costs, that is, those attributable to the musculoskeletal conditions, high among those age 45 and older, but the total medical costs they experience are also higher in these age ranges. The problems of an aging population are exacerbated by the co-occurrence of multiple chronic diseases. (Reference Table 10.9 PDF CSV).